In most heat engines, higher temperatures produce greater engine efficiency. However, the metals used in conventional internal combustion engines cannot withstand a significant increase in operating temperatures over that now used without affecting durability. But ceramics can withstand such higher temperatures and can help to retain the heat of combustion within the engine (to be extracted from the exhaust gas such as by turbocharging) to increase engine efficiency.
While ceramics have desirable high temperature properties, they generally have low material strength, distort under high temperatures, and can frequently fail or crack due to machining flaws introduced by shaping the ceramic, making their use in engines less than desirable. This is exemplified in recent government sponsored research work in Japan (see "From Japan's Labs: Ceramic Diesels", by John Hurtly, Popular Science, December, 1982, pages 94-96). In this research work, Kyoto Ceramic tried the use of silicon nitride as a flat plate to insulate the head, and silicon nitride for the entire piston, as well as for loose fitting parts such as tappets, push rod tips, and rocker arm pads. It was found that silicon nitride was not as good an insulator as some other ceramics, did not have a coefficient of thermal expansion close to conventional metal parts of an engine, and could not be fitted closely for high temperature applications. In addition, the use of only a plate of ceramic at the head permitted considerable heat to leak from the gases in the metal head passages. This same problem is presented by the disclosure of U.S. Pat. No. 4,341,826.
NGK Spark Plug tried silicon nitride as the total material for critical components of the engine (such as the head, cylinder, piston, and crankshaft) and found that in components that received high stress, ceramic is at a disadvantage if it is the load receiving member. It is significant that the use of a ceramic head was not truly explored in this patent because the engine was of a two cycle type, inducting and exhausting gases through the cylinder wall and thus requiring only a plate of ceramic to close the cylinder. See "Ceramics Parts Take Shape", by John Hartley, Automotive Industries, September, 1982, pages 56-58. As with the effort of Kyoto Ceramic, the NGK effort failed to teach how a large mass of ceramic could be used as a head with internal hot gas passages; this is understandable since silicon nitride is difficult to fabricate in thick cross-sections due to the need to migrate nitrogen gas through the material during nitridation.
NGK Insulators is reputed, in the Popular Science article, to have employed stabilized zirconia as a blanket for insulating metal parts of the engine. No information is given as to how this may be facilitated, but it is offered that stabilized zirconia possesses a coefficient of thermal expansion which is 80% of cast iron, leaving an unsatisfactory differential if the ceramic is supported for sealing by the cast iron.
To applicant's knowledge, the art has not employed ceramics as the major constituent in weight of the material of either the block or the head of an internal combustion engine due to the above indicated limitations. Several problems deter such use of ceramics, including (a) the difficulty of fabricating ceramics in large bulk with complex passages as required in an engine head or block, (b) the difficulty of maintaining adequate strength within the engine housing when ceramic is stressed and is the principal constituent, and (c) adequately compensating for differential thermal expansion between differing materials in the housing if the housing is not made entirely of the same material.